Current mirror amplifier

Abstract

Unity current gain impedance transformation at the input of a field effect transistor current mirror amplifier reduces the input impedance and renders it independent of the mirror ratio. A net saving of semiconductor material may be obtained for a given mirror ratio and input impedance when the transformation is provided by a common gate connected complementary field effect transistor meeting certain effective threshold voltage and forward transfer conductance ratio requirements.

What is claimed is: 
     
       1. A current mirror amplifier, comprising, in combination: an input terminal, a common terminal, an output terminal and a node;   first and second field effect transistors of a first conductivity type, the source electrode of each being coupled to said common terminal, the gate electrode of each and the drain electrode of the first being coupled to said node, the drain electrode of the second being coupled to said output terminal; and amplifier means having an input electrode coupled to said input terminal, an output electrode coupled to said node and a control electrode coupled to said common terminal for maintaining the impedance between said input terminal and said common terminal at a value less than the reciprocal of the transconductance of said first field effect transistor.   
     
     
       2. A current mirror amplifier, comprising, in combination: an input terminal, a common terminal, an output terminal and a node   first and second field effect transistors of a first conductivity type, the source electrode of each being coupled to said common terminal, the gate electrode of each and the drain electrode of the first being coupled to said node, the drain electrode of the second being coupled to said output terminal; and   means coupled to said input terminal, said common terminal and said node for maintaining the input impedance of said input terminal at a value less than the reciprocal of the transconductance of said first field effect transistor, said means comprising a third field effect transistor of a second conductivity type, said third field effect transistor being coupled at the source, gate and drain electrodes thereof, respectively, to said input terminal, said common terminal and said node.   
     
     
       3. The current mirror amplifier as recited in claim 2 wherein the transconductance and the threshold voltage of said third field effect transistor are greater than the transconductance and the threshold voltage of said first field effect transistor. 
     
     
       4. The current mirror amplifier as recited in claim 2 wherein the transconductance of said third field effect transistor is greater than the transconductance of said first field effect transistor and further comprising: a bias input terminal;   a substrate electrode in said third field effect transistor; and   means coupling said bias input terminal to said substrate terminal for applying a bias thereto of a value such that the threshold voltage of said third field effect transistor is greater than that of said first field effect transistor.   
     
     
       5. The combination recited in claim 2 further comprising: a further input terminal;   a fourth field effect transistor of said second conductivity type, said fourth field effect transistor being connected at the source, gate and drain electrodes thereof, respectively, to said further input terminal, said common terminal and said output terminal.   
     
     
       6. The combination recited in claim 5 further comprising: a bias input terminal;   a substrate electrode in each of said third and fourth field effect transistors; and   means coupling each said substrate electrode to said bias input terminal.   
     
     
       7. The combination recited in claim 5 further comprising circuit means for applying first and second input currents, respectively, to said input terminal and said further input terminal. 
     
     
       8. The combination recited in claim 7 wherein said circuit means comprises: current source means;   fifth and sixth field effect transistors of said second conductivity type, the source electrodes thereof being connected to said current source means, the gate electrodes thereof for receiving first and second input signal voltages; and   means coupling the drain electrode of the fifth transistor to said input terminal and the drain electrode of said sixth transistor to said further input terminal.   
     
     
       9. The combination recited in claim 5 further comprising: a current input terminal for receiving a source of current;   means coupling said current input terminal to said input terminal and said further input terminal;   a further gate in each of said third and fourth field effect transistors, each further gate for receiving a separate input signal voltage, each further gate being proximate to the source electrode of its respective field effect transistor.   
     
     
       10. A current mirror amplifier, comprising, in combination: an input terminal, a common terminal, an output terminal and a node;   first and second field effect transistors of a first conductivity type, the source electrode of each being coupled to said common terminal, the gate electrode of each and the drain electrode of the first being coupled to said node, the drain electrode of the second being coupled to said output terminal; and   a third field effect transistor of a second conductivity type, said third field effect transistor being connected at the source, gate and drain electrodes thereof, respectively, to said input terminal, said common terminal and said node.